When you really stop to consider them, human eyes are miraculous, but also quite bizarre.
They come in a range of colours – or, at least, they appear to. But what influence do these colours have on the actual functioning of the eye – and what purpose do they really serve? Why have some humans evolved to have blue eyes, others green, and others brown? Unless you’re wearing contact lenses, your eyes are unlikely to be yellow, purple, orange or red – but why is that?
Why are eyes different colours?
Human eyes come in different colours for the same reason that human skin comes in different colours. It’s all down to the amount of melanin present. If your eyes contain a lot of melanin, then less light will be reflected back out of your eye and into the world. This will give your eyes a brown appearance.
On the other hand, where there isn’t so much melanin present, light can reflect back out. And as it does this, it tends to preserve blue wavelengths – for much the same reason that the sky reflects blue wavelengths. Bear in mind that human eyes are more sensitive to blue wavelengths, so only a little bit of light-scattering needs to take place for us to perceive someone’s eyes as blue.
The amount of melanin present in your skin tends to correlate with the amount that’s in your eyes. Blue eyes are rare among dark-skinned people. This might be because dark-skinned people can trace their ancestry to parts of the world where sunlight was more plentiful, and melanin was therefore more advantageous, from an evolutionary perspective, in your eyes, hair and skin.
On the other hand, some of the earliest known people to live in Britain had blue eyes and dark skin, which just goes to show that the human body is a bit more complicated than we might assume!
Does eye colour make any difference to the way your eyes work?
When it comes to the functioning of the human body, eye colour tends to make as big a difference as skin colour: which is to say, no difference at all.
With that said, there are a few reasons to pay attention to your eye colour later in life. If it’s changing, then you might be at risk of Horner’s syndrome or pigmentary glaucoma. While these are rare, even amongst people whose eye colour changes, they’re still worth catching early.
Other myths about eyes
They say that eyes are the windows to the soul. As human beings, we’ve adapted to take a lot of social information about someone based solely on the way that their face and eyes look. In some cases, however, we make the mistake of extracting too much information. It’s possible to tell whether someone is sad or angry by looking at their eyes; it’s probably not possible to tell whether they are a Pisces.
Eye colour doesn’t change with mood – or at least, the colour of our iris doesn’t change. In response to light conditions and certain drugs, our pupils might dilate or contract, making our eyes appear darker. But they’re not actually getting darker.